My first response to this question is to say "yes," but it is difficult to just say that because repentance, from the Greek word 'metanoia,' implies the reversal of a decision. This is taken from the Igor Bible Concordance. In its fullest sense, to repent is to change your mind. Since this is an inner process, a thought process, a change of heart, would that not mean that it is a very private and personal thing? I think you can see the reason for the hesitance to come out and say, "Yes it is definitely obvious when repentance takes place."
On the other hand, if one truly changes his or her mind to the degree it would be described as "a change of heart," would it not result in changes in actions as well? So while it might not be possible to see a heart change per se, it is definitely within the realm of possibility to say that the changes in action, speech, and practice would be obvious.